The following article by Janina Radzikowska, Senior Metallographer, The Foundary
Research Institute (Instytut Odlewnictwa) Kraków, Poland was edited
by George Vander Voort, former Director, Research and Technology, Buehler Ltd..
It was originally published by Buehler in Tech-Notes, Volume 2, Issue
2 and is reproduced here with the kind permission of the Editor, Mr. Vander
Voort who granted it while still with Buehler. There are 22 accompanying photomicrographs.

Preparation of Cast Iron Foundry Alloys

Metallography in PolandEditor's Note: We are extremely pleased to be able to present to you in
this issue the superb work of Mrs. Janina Radzikowska, senior metallographer
at the Foundry Research lnstitute in Kraków, Poland. The very high
quality of her work should be an inspiration to metallographers everywhere.

Cast irons can exhibit a wide range of microstructural constituents depending
upon their composition and heat treatment. Their preparation is difficult
due to the need to properly retain the graphite phase when it is present.
The presence of micro-shrinkage cavities also presents problems, particularly
controlling bleedout of fluids and etchants. Metallographic examination may
involve only qualitative assessments, for example, to define the type and
relative size of the graphite phase and identify phases and other constituents
such as nitrides and inclusions. There has been a trend in the foundry industry
to become more quantitative with image analysis measurements of the amounts
of phases, graphite shapes, and nodule density. In our research work, we
quantify structures fully. However, most foundries do not need to be as rigorous
in quality control studies.

Preparation Procedures
For our work, which often involves quantitative measurements using image
analysis, we must faithfully reveal the true structure. Thus, our preparation
procedure is more elaborate than that used by many foundries. We utilize
color tint-etching methods extensively, and these require a very high-quality
preparation practice.

For the work shown here, I used the following procedure with slight modifications
depending upon whether or not the specimen was to be etched and if a tint
etch was to be used. Mounted specimens were placed in a holder for six (usually)
specimens. Central force and complementary rotation (platen and holder rotating
in same direction) were used. The steps were:

METADI® Fluid was used as the lubricant/extender with the
diamond abrasives. After each polishing step (Nos. 3, 4, 5 and 7), the specimens
were washed with alcohol and blown dry with compressed air (which must be
clean and dry). Washing with water sometimes results in corrosion stains
on the surface, so I usually use only alcohol for the final cleaning step
(No. 7). lndividual force can also be used, especially if I do not have enough
specimens to fill the holder (divide the force values given above by 6 to
determine the individual force to use).

Microstructures
One should always begin microstructural investigations by examining the
as-polished specimen before etching. This is a necessity, of course, for
cast iron specimens if we are to properly examine the graphite phase. Brightfield
vertical illumination will be our starting point, but the benefits of crossed
polarized light will also be explored.

Cast irons with a composition equivalent to about 4.3%
C solidify as a eutectic. Because cast irons are not simple binary Fe-C alloys,
it is usual practice to calculate the carbon equivalent (CE) value which
is the total carbon content plus one-third the sum of the silicon and phosphorus
contents. If the CE is > 4.3, it is hypereutectic; if it is < 4.3,
it is hypoeutectic. Table 1 lists the CE values
and compositions for each cast iron shown in this issue.

In the Fe-C system, the carbon may exist as
either cementite, Fe,C, or as graphite. So the eutectic reaction is either
liquid transforming to austenite and cementite at about 1130°C or liquid
transforming to austenite and graphite at about 1135°C. Addition of
elements such as silicon promote graphite formation. Slow cooling rates promote
graphite formation, while higher rates promote cementite. The eutectic grows
in a cellular manner with the cell size varying with cooling rate which
influences mechanical properties.

Gray lron
Figure 1 shows interdendritic flake graphite in a hypoeutectic alloy (see
Table 1 for composition of alloys) where proeutectic
austenite forms before the eutectic reaction. This type of graphite has been
given many names. In the US it is referred to as Type D (ASTM A247) or as
undercooled graphite. It was thought that the fine size of the graphite might
be useful, but it is not technically useful as it always freezes last into
a weak interdendritic network.

Other graphite forms are also observed. For example, Figure 4 shows disheveled
graphite flakes in a casting. Note that a few nodules are present. This appears
to be a mix of B- and D-type flakes. Figure 5 shows a hypereutectic gray
iron where very coarse flakes form before the eutectic which is very fine.
This is similar to C-type graphite.

Nodular Iron
The addition of magnesium ('inoculation') desulfurizes the iron and causes
the graphite to grow as nodules rather than flakes. Moreover, mechanical
properties are greatly improved over gray iron; hence, nodular iron is widely
known as'ductile iron'.

Nodule size and shape perfection can vary depending upon composition and
cooling rate. Figure 6 shows fine nodules, about 15-30µm in diameter,
while Figure 7 shows coarser nodules (about 30-60µm diameter) in two
ductile iron casts (see Table 1 for compositions).
Note that the number of nodules per unit area is much different, about 350
per mm2 vs.125 per mm2, respectively.

Compacted Graphite
Compacted graphite is a more recent development made in an effort to improve
the mechanical properties of flake gray iron. Figure 8 shows an example where
the longest flakes are in the 60-120µm length range. Compare these flakes
to those shown in Figures 2 and 3.